The present invention relates to the investigation of particle characteristics, particularly the distribution of these characteristics over a set of particles, and relates in particular to a device for measuring the apparent density distribution of a set of particles that can implement a flotation process.
The creation and practical use of particulate materials are the first steps in manufacturing numerous composite materials. The simplest of these materials are comprised of two components: the particles, and a binder. The application properties of these materials often depend on the microstructure of the elementary particles employed.
Devices are known for measuring the density of a liquid using particles with a known density such as the device described in patent DE820981.
Devices are also known for measuring the density of a mixture of liquid and solid particles such as that described in U.S. Pat. No. 1,344,370. However, these devices do not measure particle density.
The apparent density of the particles can be an important microstructure parameter. For example, it enables the homogeneity of the particles to be characterized quantitatively and their degree of purity or fault density to be determined. The distribution of the apparent density of a set of particles enables the homogeneity of the particle set to be characterized.
The apparent density of a particle is determined by the ratio between its mass and its apparent volume. There are two major groups of methods for measuring particle density: indirect methods and direct methods.
The indirect methods are based on measuring the mass and volume of the particles. They are gas or liquid pycnometer methods. The masses are determined by weighing. The volumes are determined by pressure measurements (gas) or weighing (liquid). These methods give access to the average density of the set of particles. However they do not enable the distribution of the apparent densities over a set of particles to be determined.
The direct methods are based on decanting, also called flotation, within a liquid whose density is being measured. These methods separate the particles denser than the liquid in the lower part from the particles less dense than the liquid in the upper part. Two techniques are used: the density gradient method and the flotation method.
The density gradient method, implemented in particular in U.S. Pat. No. 4,290,300, consists of creating a density gradient in a liquid column. The particles are then released at the top of the column. They descend in the column until the density of the liquid balances the density of the particle. Prior calibration of the column enables the liquid density and column height to be linked. When the stabilization height of each particle is recorded, the apparent density distribution of the particles of a given set can be obtained.
The flotation method consists simply of submerging a particle sample in a liquid of known density, decanting the particles that are denser than the liquid, and collecting and weighing these particles. The principle of this measurement is simple but implementation for an accurate and reproducible result is more delicate. The difficulties of this type of method are the accuracy of monitoring the density of the flotation liquid and extracting fractions of particles deposited at the lower part of the container.
The particle density measuring devices known to date using a flotation separation method involve several major disadvantages, namely separation of the different classes of particles with different densities is not distinct, and recovery of the particles produces recirculation of the flotation liquid, which interferes with extraction and hence with measurement unless additional techniques are used such as the pressure exerted by a fluid on the flotation liquid or centrifugation—techniques that are hazardous if applied to particles of an explosive material.
Centrifugation measuring devices are also known; however, their use involves safety problems in the case of particles of high-energy materials.